Abstract

Background and Purpose—Here, we assessed how sustained is reversal of the acute diffusion lesion (RAD) observed 24 hours after intravenous thrombolysis, and the relationships between RAD fate and early neurological improvement.

Methods—We analyzed 155 consecutive patients thrombolyzed intravenously 152 minutes (median) after stroke onset and who underwent 3 MR sessions: 1 before and 2 after treatment (median times from onset, 25.6 and 54.3 hours, respectively). Using voxel-based analysis of diffusion-weighted imaging (DWI)1, DWI2, and DWI3 lesions on coregistered image data sets, we assessed the outcome of RAD voxels (hyperintense on DWI1 but not on DWI2) as transient or sustained on DWI3, and their relationships with early neurological improvement, defined as ΔNational Institutes of Health Stroke Scale ≥8 or National Institutes of Health Stroke Scale ≤1 at 24 hours. Tmax and apparent diffusion coefficient values were compared between sustained and transient RAD voxels.

Introduction

All current MR-based approaches in candidates for reperfusion therapy depend on diffusion-weighted imaging (DWI) for an accurate assessment of infarct core. DWI is necessary not only to map the perfusion weighted imaging–DWI mismatch but also in relation to lesion volume cut-offs above which thrombolysis may not have clinical benefit or be even harmful. However, the widely prevalent model that at early time points, the DWI lesion exclusively represents the infarct core has been a matter of controversy. Both early animal research1–3 and recent clinical studies4–8 have reported that DWI lesions can reverse either spontaneously or after recanalization, suggesting that they contain not only core but also some penumbral tissue. Thus, DWI lesion reversal has been reported in one third of patients after recanalization therapy9 and in up to one half after intravenous thrombolysis.6 In the latter population, DWI lesion reversal at 24 hours was associated with early neurological improvement (ENI) in a dose-dependent manner,6 and with favorable clinical outcome at hospital discharge in another study,8 supporting the notion that early reperfusion and its impact on the 24-hour DWI lesion are potential markers of therapeutic efficacy and favorable outcome.

One could however argue that when assessed 24 hours after therapy, DWI lesion reversal may be largely transient and regularly followed by secondary injury. Effectively, both transient and sustained DWI reversal have been reported in experimental and clinical stroke.3,9–15 The fact that early reperfusion mediates DWI reversibility in experimental3,10–13 and human stroke7,8,16–18 could explain why sustained DWI reversal is rather uncommon when treatment is administered in delayed timeframes,9,19,20 whereas being common and sizeable when treatment is given at earlier time points.6,8,18 However, the full temporal profile of this phenomenon remains largely unknown, especially after intravenous thrombolysis.

Taking together the above data, we make the hypothesis that most of the DWI lesion reversal observed 24 hours after intravenous thrombolysis administered ≤4.5 hours from onset will be sustained on follow-up MR. We further predict that sustained, but not transient, DWI lesion reversal is associated with ENI and has less impaired baseline apparent diffusion coefficient (ADC) and perfusion. To test these hypotheses, we assessed voxel-wise the fate of early DWI lesion reversal, namely transient or sustained, in a series of patients with acute stroke imaged before, on the following day, and 2 to 7 days after intravenous thrombolysis, and their respective relationships with ENI.

Materials and Methods

Patients

This was a retrospective analysis based on a prospectively collected registry of patients treated by intravenous thrombolysis for ischemic stroke from January 2009 to December 2013. The study was approved by the local ethics committee. In our center, MR examinations are scheduled at 3 time points: before treatment as first-line diagnostic tool (MR1), on the following day (to assess the arterial recanalization status, and search for complications; MR2) and at 48 to 72 hours (MR3) for pathogenic work-up (cervical MR angiography), and final infarct assessment (DWI only). Here, we included all patients who fulfilled the following inclusion criteria: (1) final diagnosis of ischemic stroke; (2) treated exclusively with intravenous tissue-type plasminogen activator; (3) who underwent the 3 aforementioned MR exams within 7 days after stroke onset; and (4) without severe DWI artifacts.

We prospectively recorded demographic details, stroke onset-to-MRI and onset-to-treatment times, National Institutes of Health Stroke Scale at admission and 24 hours, blood pressures, serum glucose, stroke arterial distribution, and hemorrhagic transformation on MR2 according to the European Cooperative Acute Stroke Study classification criteria from the T2* sequence. Pretreatment occlusion was categorized into proximal (carotid, basilar artery, or proximal post-Willisian segment) and distal or no occlusion. Twenty-hour recanalization was defined as thrombolysis in cerebral infarction criteria ≥2 on MR2 angiography. ENI was defined as a gain of National Institutes of Health Stroke Scale ≥8 within the first 24 hours, or National Institutes of Health Stroke Scale ≤1 at 24 hours.21,22 Modified Rankin Scale was assessed at 3 months.

Image Analysis

Coregistration and segmentation. MR2 and MR3 images were coregistered to MR1 using an automated 3-dimensional rigid registration with FSL software-FLIRT (Oxford, England), visually checked then manually corrected whenever necessary. DWI1, DWI2, and DWI3 lesions were manually segmented using MANGO software version 3.0.4. Regions of interest were drawn using a combination of interactive thresholding and drawing tools. The segmented DWI voxels were projected onto apparent ADC maps to exclude areas of T2 shine-through effects and include areas of decreased ADC with subtle DWI signal changes. Hemorrhagic transformations were manually included in the segmentation of DWI2 and DWI3 to avoid overestimating any reversal of the DWI lesions.

DWI lesion reversal. Reversible acute DWI (RAD) voxels were defined as those DWI1 voxels that did not overlap with the coregistered lesion on DWI2.6,24 To assess the fate of RAD voxels, we distinguished sustained RAD (sRAD) defined as RAD voxels that did not overlap with the DWI3 lesion and transient RAD (tRAD) defined as RAD voxels that overlapped with the DWI3 lesion. The unchanged area corresponded to voxels that were hyperintense in all 3 DWI scans (Figure 1). To rule out the possibility that the DWI3 signal had normalized while corresponding T2-changes appeared, we visually checked that sRADs were not hyperintense on T2-EPI, and that T2-EPI signal changes differed between sRAD and unchanged areas on the last MR.

We then computed the %sRAD (volume of sRAD/RAD×100) in the whole population and in subgroups of patients with sizeable RAD using 3 definitions (Table 1). These subgroup analyses were performed to rule out potential measurements errors linked to small RAD volumes.6 For the same reasons, baseline ADC and Tmax in sRAD, tRAD and unchanged 3-dimensional regions of interests were computed only for the 50% of patients with RAD volume above the median value.

Statistical Analysis

Continuous variables were described as mean±SD or median and interquartile range, and compared with Student t test or Mann–Whitney U test, as appropriate. Categorical variables were compared using χ2 or Fisher exact test as appropriate. To test our hypothesis that sRAD volume may be a correlate for clinical outcome, we first conducted a univariate analysis comparing clinical and imaging variables between patients with and without ENI. In a multivariate analysis using a backward selection method (entry for P<0.20), we then tested the association of sRAD volume and ENI (MedCalc, 13.3.3.0; Mariakerke, Belgium).

Results

Patients

During the study period, 354 patients underwent intravenous thrombolysis for an ischemic stroke in our center, of which 199 (56.2%) were excluded because of bridging therapy (n=54), use of computed tomography instead of MR1 (n=38) or MR2 (n=28), lack of MR3 (n=52) or DWI3 (n=7), MR3 obtained >7 days (n=12), severe artifacts on DWI (n=5) or death before follow-up MR (n=3). Excluded patients did not differ from the included patients in age or onset-to-treatment time but had higher baseline National Institutes of Health Stroke Scale (median [interquartile range], 14 [8–20] versus 11 [7–18]); P=0.01). In the 155 included patients, the median (interquartile range) onset-to-treatment time was 152 minutes (120–190), and time from onset-to-MR1, MR2, and MR3 were 2.0 hours (1.5–2.6), 25.6 hours (21.5–29.1), and 54.3 hours (44.2–75.1), respectively. Lesion volumes on DWI1, DWI2, and DWI3 were 15.6 (6.0–50.9) mL, 28.9 (13.5–98.6) mL, and 34.0 mL (13.0–106.0 mL; P=0.43 relative to DWI2), respectively. Baseline perfusion weighted imaging was available in 96 (61.9%) patients.

sRAD Versus tRAD

Across the whole sample, median (interquartile range) RAD volume at MR2 was 2.8 (1.1–6.6) mL and >5 and 10 mL in 54 (34.8%) and 23 (14.8%) patients, respectively. As shown in Table 1, median sRAD represented 70.8% (51.6%–88.2%) of the pooled RAD voxels, and >80% of the pooled voxels belonging to sizeable RAD, regardless of the definition. Similar results were obtained when analysis was conducted per-patient rather than on pooled RAD voxels (data not shown). Sixteen (10.3%) patients had sRAD ≥10 mL, and in 20 patients (12.9%) sRAD involved more than half of the baseline DWI1 lesion. On the last MR, no sRAD areas were hyperintense on T2-EPI, and signal changes on T2-EPI were significantly lower in sRAD than in infarct core (P<0.0001; Figure 2).

Sustained diffusion-weighted imaging (DWI) reversal. DWI1 showed a hyperintense lesion in the left middle cerebral artery territory in this patient presenting with right hemiparesis and dysphasia (National Institutes of Health Stroke Scale [NIHSS]=12). After thrombolysis, the whole DWI lesion reversal observed on DWI2 was sustained on DWI3, without corresponding signal changes on T2-weighted images. The 24-hour NIHSS was 0.

In the subset of patients with RAD volume above the median value (ie, 2.8 mL), baseline Tmax (available in 51 [65.4%] patients) was significantly less impaired in sRAD than in tRAD, and in tRAD than in the unchanged area. sRAD showed a nonsignificant trend (P=0.08) for higher baseline ADC than tRAD, and both sRAD and tRAD had higher ADC than the unchanged area (Table I in the online-only Data Supplement). These results were not substantially different using alternative definitions of sizeable RAD (eg, >5 or 10 mL; data not shown).

Discussion

Addressing the time course of DWI lesion reversal in a sample of thrombolyzed patients, we showed that the reversibility observed at 24 hours was sustained at the final MR session in over two third of RAD voxels. As predicted, only sustained reversibility was associated with ENI, independently of other relevant variables. Finally, initial hypoperfusion was, as also predicted, worse for tRAD voxels. These findings shed new light on the currently debated issues around transient versus sustained DWI lesions after thrombolysis.

That most of the DWI lesion reversal observed at 24 hours exhibits sustained tissue salvage in the early thrombolysis time window is not unexpected. Indeed, early reperfusion is of utmost importance for sustained tissue salvage, with studies demonstrating permanent normalization of parts or all of the initial DWI lesion after short periods of occlusion.7,18,25,26 For instance, in rodents, resolution of DWI lesions is sustained after short, but transient after long-lasting occlusions.3,13 The established relationship between early recanalization and sustained lesion reversibility3,26,27 partly explains why reversibility is often transient in patients treated in later time windows.9,19,20,28 Differences between studies on the outcome of DWI reversibility may also be explained by heterogeneity in imaging protocol.18 The timing of follow-up MRI is indeed essential when evaluating DWI lesion reversal. In rats with prolonged occlusion, the ADC decreases significantly during occlusion, recovers completely at 60 minutes after reperfusion, but declines secondarily at 12 hours after reperfusion.3 Thus, transient DWI lesion reversal occurs rapidly after reperfusion and would only be captured if follow-up MR is scheduled soon after treatment. Indeed, prominent transient DWI lesion reversal has been observed when the first follow-up MR was scheduled soon (<12 hours) after treatment.9,18,20 Conversely, 24-hour follow-up MR likely misses most of this temporary postreperfusion reversal because images are acquired beyond the time of transient ADC pseudonormalization. This in turn explains why most of the DWI lesion reversal that we observed on 24 hours MR was sustained, in line with a recent study on patients with minor to moderately severe stroke, 20% of whom received intravenous tissue-type plasminogen activator, that found 87% of the DWI lesion reversal on 24-hour MRI to be sustained at 4 to 7 days.15 On a pathophysiological standpoint, sRAD voxels were initially less severely hypoperfused than their transient counterparts, in line with our initial hypothesis. Our Tmax data suggest that perfusion could help distinguish true core from the penumbral portion of the acute DWI lesion.

The clinical implications of DWI lesion reversal and of its sustained versus transient compartments remain debated.28 The phenomenon of transient lesion reversal has been speculated to reflect salvage followed by secondary injury.20 However, the expected clinical correlate of improvement followed by deterioration has never been observed in this scenario.18,28 Moreover, transient reversibility seems unrelated to ENI in patients treated in the early time window as shown here, congruent with findings in the later time windows.9,19 This lack of clinical correlate suggests that the transient pseudonormalization of DWI signal does not indicate full tissue recovery from ischemic injury, in line with animal studies.3 Thus, there may be an apparent DWI reversibility (tRAD), which is transient because the underlying tissue is damaged early after the ischemic insult, and a real reversibility (sRAD), which corresponds to salvaged penumbra, and hence has clinical correlates.

We found sizeable sRAD in 10% of patients, involving >50% of the initial DWI lesion in 12.9%, indicating that although large sustained DWI lesion reversal is not common, it is not negligible either in the early timeframe. These findings mirror the rate and extent of sRAD observed in 60 patients from the 3- to 6-hour window Diffusion Weighted Imaging Evaluation for Understanding Stroke Evolution Study-2 (DEFUSE 2),9 namely >10 mL in 17% patients. In DEFUSE, spontaneously reperfused DWI lesions before thrombolysis had the highest rate of sustained DWI reversal, and the latter was associated with clinical recovery.7 Here, we found that the volume of sustained, but not that of transient, DWI lesion reversal was strongly associated with ENI. This finding reinforces the notion that sustained reversibility after thrombolysis is not an epiphenomenon and that it does have meaningful clinical correlates. That the sRAD volume is a correlate of ENI does not however imply that it alone accounts for the latter. Although ENI largely derives from concomitant reperfusion of adjacent mismatch regions, it may in part result from reperfusion of the DWI lesion itself in case of large sRAD volumes. Given that poststroke neurological improvement is underpinned by salvaged penumbra,29 even relatively small sRADs in peri-infarct areas might also play a clinical role.

The use of multimodal MR in evaluating efficacy of thrombolytic therapy has been recently emphasized in a study showing that early reperfusion at 2 hours and decrease in DWI lesion volume at 24 hours were independent predictors of favorable outcome.8 Another study also found dramatic improvement to be more common in patients with early DWI lesion reduction after intravenous thrombolysis.18 Note however that both studies were based on DWI lesion volume subtraction, as compared with voxel-wise here. That the 24-hour reversibility was largely sustained using serial coregistered MRIs here strengthens the notion that 24-hour follow-up MR affords a reasonable estimate of sustained DWI lesion reversibility, itself a surrogate for early recanalization and ENI after thrombolysis.8

Our study has several limitations. First, that 3 MR examinations were required likely explains why eligible patients had less severe strokes. This may have biased our results toward favorable outcome and caused some overestimation of the RAD phenomenon. However, the volumes of total RAD and sRAD were in the previously reported ranges.6,9,19 Second, the timing of MR3 was variable and delayed changes might have been missed, especially if MR3 was performed too early. However, post hoc analyses showed that patients whose MR3 was the earliest (ie, lower onset-to-MR3 time quartile) exhibited a similar %sRAD than those with latest MR3 (ie, upper quartile) (75.5% [57.5%–90.5%] versus 69.0% [57.0%–90.0%]; P=0.79). Although the timing of MR3 minimized the confounding effect of vasogenic edema, a rim of edema may nevertheless develop early in large lesions and lead to overestimation of the final lesion. This would in turn have led to an underestimation of sRAD and if anything weakened our main findings. Finally, a normal DWI signal does not exclude the presence of subtle histological changes undetectable on standard MRI sequences.3,13,30

In summary, we showed that most of the diffusion lesion reversibility captured on 24-hour imaging is effectively sustained at later MR scans, and that sustained reversal is an imaging correlate of ENI. This strengthens the notion that 24-hour MR follow-up affords a reasonable estimate of sustained DWI lesion reversibility, itself an imaging correlate of early recanalization after thrombolysis.8